Silicon, the second most abundant element in Earth’s crust, forms the foundational material for modern electronics and a vast array of industrial products. The element exists most commonly in nature as silicon dioxide (\(\text{SiO}_2\)), known as silica or quartz. Extracting the pure elemental silicon (Si) from this abundant raw material requires a series of highly energy-intensive and complex refinement stages. The final purity level dictates whether the silicon is suitable for simple industrial applications or the highly demanding world of semiconductor technology.
Producing Metallurgical Grade Silicon
The production of nearly all silicon begins with the creation of Metallurgical Grade Silicon (MGS) using a carbothermic reduction process. This process involves heating silica (quartz) to extremely high temperatures with a carbon source. Raw materials include high-purity quartz and carbonaceous reductants, such as coal, coke, charcoal, or wood chips.
The mixture is smelted in a submerged arc furnace, where temperatures can reach over \(2,000\) degrees Celsius. The carbon reacts with the oxygen in the silica, reducing the silicon dioxide to elemental silicon and releasing carbon monoxide gas. This industrial smelting yields MGS, which has a purity of about \(98\) to \(99\) percent. While this purity level is sufficient for many industrial uses, the remaining \(1\) to \(2\) percent of impurities, including iron, aluminum, and calcium, make it unsuitable for microelectronics.
Chemical Purification for Electronic Use
The transition from MGS to the hyper-pure material required for electronics necessitates the complex chlorosilane purification process. MGS is ground and reacted with hydrogen chloride gas, converting the solid material into a volatile liquid compound, most commonly Trichlorosilane (\(\text{SiHCl}_3\)), or TCS. This conversion is important because the liquid TCS has a relatively low boiling point, allowing impurities to be separated through distillation.
The crude TCS is subjected to multiple stages of fractional distillation in tall columns. This physical separation removes the majority of non-silicon impurities, which have different boiling points than the TCS. Following distillation, the ultra-pure TCS is converted back into solid silicon through the Siemens process.
In the Siemens process, the purified gaseous TCS is introduced into a high-temperature reactor, mixed with hydrogen, and decomposed at temperatures around \(1,100\) degrees Celsius. This chemical vapor deposition causes the pure silicon to deposit onto thin, heated silicon rods, which grow into large Polysilicon rods. This polycrystalline material, known as Semiconductor Grade Silicon (SGS), achieves the required purity level, often exceeding \(99.9999999\) percent, or “nine nines” purity.
Growing Single Crystal Wafers
The hyper-pure polysilicon must be transformed into a single crystal structure to function as a semiconductor substrate. This physical structuring is achieved through the Czochralski (Cz) method. The process begins by melting the polysilicon feedstock in a quartz crucible at a temperature slightly above the silicon melting point of \(1,414\) degrees Celsius.
A small seed crystal is dipped into the molten silicon and then slowly withdrawn while simultaneously being rotated. By precisely controlling the temperature, pull rate, and rotation speed, the liquid silicon solidifies onto the seed crystal, adopting its exact atomic orientation. This process forms a large, cylindrical ingot, often called a boule, that can be up to \(300\) millimeters in diameter and over a meter in length.
The monocrystalline structure is necessary because any grain boundaries or structural defects would interfere with the controlled flow of electrons, rendering the material useless for microchip fabrication. Once the boule is grown, it is ground to a precise diameter, and a diamond-embedded wire saw slices it into extremely thin discs, which are the raw silicon wafers. These wafers undergo final steps of lapping, etching, and mechanical-chemical polishing to create the mirror-smooth surface necessary for photolithography and circuit printing.
Diverse Applications of Silicon Grades
The complex and expensive multi-stage purification is only justified for high-technology applications. The lower-purity Metallurgical Grade Silicon (MGS) is widely used in the metallurgical industry, primarily as an alloying agent to improve metal properties. Large quantities of MGS are added to aluminum to enhance its castability, strength, and wear resistance for automotive parts and structural components.
MGS is also the precursor for the chemical industry, used to produce silicones, silanes, and other compounds found in sealants, lubricants, and plastics. Conversely, Semiconductor Grade Silicon (SGS) is reserved for the electronics and solar industries. This hyper-pure material forms the substrate for integrated circuits, microprocessors, memory chips, and high-efficiency photovoltaic cells.